In light of the problem of ozone layer depletion in recent years, the restrictions on CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons) that are used as refrigerants in conventional refrigerating air conditioners have become more stringent, and HFCs (hydrofluorocarbons) are coming into use as substitute refrigerants. However, HFC refrigerants are also associated with problems such as increased contribution to global warming, and the use of natural refrigerants as substitutes for such fluorocarbon refrigerants is currently being researched. Among such refrigerants, carbon dioxide refrigerants are known to be harmless to the environment and highly safe, while also having advantages such as compatibility with oils and machine materials and being readily available. Research has also recently begun on their use as refrigerants for automobile air conditioners that employ open type compressors or hermetic type electrical compressors.
Esters which are compatible with HFC refrigerants, carbonic acid esters, PAG (polyalkylene glycols), polyvinyl ethers and the like have been either investigated or employed as refrigerating machine oils for HFC refrigerants (see Patent documents 1-10, for example). Also, ester-based refrigerating machine oils, for example, are used as refrigerating machine oils for carbon dioxide refrigerants (see Patent document 11, for example).
As a goal in many fields in recent years continues to be that of increasing energy savings, efforts have been directed toward achieving energy savings in the field of refrigerating air conditioners as well, by improving thermal efficiency and reducing power consumption. Techniques have been proposed for improving energy efficiency by lowering the viscosity of refrigerating machine oils, as an energy saving strategy from the viewpoint of the refrigerating machine oil (see Patent documents 12 and 13, for example).    [Patent document 1] Published Japanese Translation of PCT Application HEI No. 3-505602    [Patent document 2] Japanese Patent Application Laid-Open HEI No. 3-88892    [Patent document 3] Japanese Patent Application Laid-Open HEI No. 3-128991    [Patent document 4] Japanese Patent Application Laid-Open HEI No. 3-128992    [Patent document 5] Japanese Patent Application Laid-Open HEI No. 3-200895    [Patent document 6] Japanese Patent Application Laid-Open HEI No. 3-227397    [Patent document 7] Japanese Patent Application Laid-Open HEI No. 4-20597    [Patent document 8] Japanese Patent Application Laid-Open HEI No. 4-72390    [Patent document 9] Japanese Patent Application Laid-Open HEI No. 4-218592    [Patent document 10] Japanese Patent Application Laid-Open HEI No. 4-249593    [Patent document 11] Japanese Patent Application Laid-Open No. 2000-104084    [Patent document 12] Japanese Patent Application Laid-Open HEI No. 10-204458    [Patent document 13] Japanese Patent Application Laid-Open No. 2000-297753